1. Field of the Invention
This invention relates to a thin film of metal phase on graphite. More particularly, this invention relates to a thin film of chemically wetted metal phase on graphite and a process for preparing same wherein said metal is selected from the group consisting essentially of Ni, Co, Mo and mixtures thereof.
2. Summary of the Invention
A composition of matter has been discovered which comprises a thin film of chemically wetted metal phase on graphite wherein said metal is selected from the group consisting essentially of Ni, Co, Mo and mixtures thereof. By chemically wetted is meant that the metal wets and chemically bonds to the surface of the channels in the graphite. While not wishing to be held to any particular theory, it is believed that the metal films of this invention are approximately one monolayer thick. These films are unique compositions of matter in themselves inasmuch as they exhibit strong metal-support interaction and possess properties other than those of the bulk metal. The term "metal phase" thus refers to those unique films. These films are useful as precursors in forming dispersions of the metal on the graphite support. It is well known in the art that dispersions of these metals on a graphite support are known to be useful catalysts, such as hydrogenation catalysts.
The compositions of this invention are formed via a process which comprises the sequential steps of (a) heating a composite of said metal and graphite in an inert, hydrogen-containing atmosphere at a temperature of from about 800.degree.-975.degree. C. for a time sufficient for the metal to form a plurality of metal-containing channels in the graphite, and (b) heating the metal-containing, channeled composite formed in (a) to a temperature of at least about 975.degree. C. in an inert, hydrogen-containing atmosphere for a time sufficient for the metal in said channels to spread out and chemically wet at least a portion of the surface of said channels as a thin film of metal phase. Preferred metals are Ni and Co and a particularly preferred metal is Ni.
By graphite is meant graphite or mixture of graphitic and non-graphitic material. This includes relatively pure forms of a graphite such as graphite single crystals and Grafoil as well as mixtures of graphite with other materials. Illustrative but non-limiting examples of mixtures of graphite with other carbonaceous materials include asphalt, pitch, coke formed as a result of various hydrocarbon conversion reactions in petroleum refineries and petrochemical plants, etc., as well as coke formed on catalysts containing Ni, Co, Mo and mixtures thereof. As is well known to those skilled in the art, crystalline forms of carbon such as graphite have a basal plane or a-face (&lt;1120&gt;direction) plane and a plane perpendicular to the basal plane which is a c-face (&lt;1010&gt;direction). In the process of this invention, particles of the metal create channels in the c-face parallel to the a-face by catalytically gasifying the graphite with hydrogen. This increases the surface area of the c-face. It has been found that the metal will channel into the c-face surface and chemically wet the soformed channels, but will not channel into the a-face or basal planes. It has also been found that if the graphite is mixed with non-graphitic or amorphous carbon, the channeling metal particles will continue to channel into and gasify the amorphous carbon. Thus, the channeling metal particles will gasify a mixture of graphite and non-graphitic or amorphous carbon.
As has heretofore been stated, metals that have been found to be useful for the composition of matter of this invention are Ni, Co, Mo and mixtures thereof. Nickel and cobalt are preferred and nickel is particularly preferred as the metal. It is understood, of course, that the process of forming the catalyst of this invention may start with a composite of the metal and graphite or graphite-containing material. Illustrative, but non-limiting examples include coke deposited on a metal surface containing one or more of said metals, such as coked steam cracker tubes, coked catalysts, etc. Alternatively, the metal may be added to the graphite or graphite-containing support by any convenient means known to those skilled in the art. Illustrative, but non-limiting examples include evaporating the metal onto the graphite in a vacuum, plasma or flame spraying the metal onto the support and various wet chemistry techniques employing metal precursors such as impregnation, incipient wetness, etc., followed by drying the contacting with a reducing atmosphere at elevated temperature to insure that the deposited metal is in the reduced, metallic form. Reducing the metal may be part of the heating step of the process wherein the composite is heated in a hydrogen atmosphere to form metal-containing channels in the graphite support. Metal precursors may be initially present on the graphite in the form of a metal salt or oxide such as carbonate, bicarbonate, sulfate, nitrate, etc., the main criterion being that the metal precursor be capable of decomposing to or being reduced to the metal at a temperature below about 875.degree. C. and preferably below about 800.degree. C.
The metal-graphite composite must be heated in an inert, hydrogen-containing atmosphere at a temperature within the range of from about 800.degree.-975.degree. C. for a time sufficient for the metal to form a plurality of metal-containing channels in the graphite. By inert, hydrogen-containing atmosphere is meant an atmosphere that is net reducing to both the metal and the graphite and which will not adversely affect either the graphite support, the metal, or the gasification reaction. Enough hydrogen must be present to catalytically gasify and channel the graphite. The hydrogen may be initially present therein or it may be formed, in-situ by using a mixture of, for example, steam and ethane and other mixtures of steam and saturated hydrocarbons such as paraffins and saturated cyclic hydrocarbons. The temperature range for channeling is critical inasmuch as channels will not be formed at temperatures below about 800.degree. C. At temperatures above about 975.degree. C., in an inert, hydrogen-containing atmosphere, the metal will spread out and chemically wet the channels as a thin film at which point catalytic gasification and channeling cease. Channeling temperatures of from about 800.degree.-975.degree. C. are preferred and particularly preferred are temperatures within the range of from about 800.degree.-925.degree. C.
When the metal channels into the c-face of the graphite, it does so by catalytically gasifying the carbon with hydrogen to form a gas such as methane. The figure schematically illustrates gasification and channeling of the graphite by a globule of nickel of about 500.ANG. in diameter. In a preferred embodiment of the invention, the metal-graphite composite will be heated within this temperature range in an inert, hydrogen-containing atmosphere for a time sufficient to achieve from about 5-20 wt. % gasification of the graphite support. Unless catalytic gasification of the graphite or graphite-amorphous carbon mixture is the desired result it is preferred that the total catalytic gasification of the graphite due to the channeling not exceed about 25 wt. % of the graphite. In practice, it has been found that the gasification rate of the graphite is roughly proportional to the concentration of metal thereon up to about 5 wt % metal. As the amount of metal on the graphite exceeds about 5 wt %, the gasification rate approaches a constant value.
After channeling of the graphite support has proceeded to the desired level, as evidenced by the amount of gasification of the graphite, the temperature is raised above about 975.degree. C. at which point the metal in the channels spreads out and chemically wets the surface of the so-formed channels as a film of metal phase and catalytic gasification ceases. As stated before, it is believed that the metal chemically wets the channels as a film approximately one monolayer thick. The metal film exhibits strong interaction with the graphite support and is in itself a unique composition of matter inasmuch as it does not exhibit the properties of the bulk metal. Thus, the term "metal phase" refers to this unique film. In order for this metal-wetting to occur, it is important that the metal-graphite composite be in contact with an inert, hydrogen-containing atmosphere. This atmosphere must be net reducing with respect to both the metal and graphite support. A preferred temperature range for the wetting and metal phase film forming step will range from about 975.degree. to 1150.degree. C., the upper limit being governed by noncatalytic gasification of the graphite which begins to occur at about 1200.degree. C. in the presence of hydrogen. However, if necessary, one can exceed the upper limit of 1150.degree. C. without adversely effecting the metal wetted surface of the composite. One merely loses more graphite support.